Never too late, I know a gentlemen at the gym where I work out at who is now in his 70's and a few years ago, just completed his first IM(Hawaii). He got into tri's in his 60's. Good luck in your training and your race!
IM Wisconsin Registration is the day after the race at 9 AM. On-line registration is available at 11AM (both Central Standard Time). I would recommend coming to next years event to sign up for 2009 to ensure you get a spot.
Jb4fun is right. Registration starts at 9 am the day after the event. I got there at 6:40 yesterday and I was probably around the 300th person outside in line...51 degrees and raining. They opened the doors to Monona Terrace at 7 am and some of us were able to sit inside. When they started letting people register at 8:30, there were around another 1000 or more people behind me, out the door and along the side of the Terrace, in the rain.
IM Wisconsin is well run, and the volunteers are amazing. Having much of the course run through Madison, there are thousands of spectators lending support for most of the run and the bike is well supported, too.